The present invention relates to low-noise preamplifiers and, more particularly, to a novel low-noise preamplifier circuit utilizing a plurality of bipolar transistors in radio-frequency (RF) parallel connection, particularly for use in a nuclear magnetic resonance (NMR) imaging/spectroscopy system, and the like, with a response signal reception antenna having an impedance not matched to a system standard characteristic impedance.
It is well known that the amplitude of received RF signals, particularly in nuclear magnetic resonance (NMR) imaging and spectroscopy experiments, can be relatively small. If high-gain amplifiers are provided to increase the signal amplitude, the signal-to-noise ratio becomes a prime consideration; the high-gain amplifier must contribute relatively little noise power. Thus, a low-noise, high-gain preamplifier must add as small an amount of extra noise power as possible; this is equivalent to requiring that the noise factor, noise figure or effective temperature of the preamplifier to be as low, i.e. as close to 1, 0 dB. or 0.degree. K., respectively, as possible. As a preamplifier has not yet been build which does not contribute some additional noise power at its output, a non-zero noise contribution will be provided by the preamplifier in any receiving system. While it is desirable to provide a noise level below some maximum level (and to provide such a low-noise preamplifier at the lowest possible cost) cost and noise level must be balanced against an associated minimum preamplifier gain (necessary for overcoming the noise contributions of subsequent receive subsystem stages) which must be present over a certain minimum useful frequency bandwidth. In an NMR system, as an illustrative example, the frequency characteristics of the preamplifier are thus dependent upon the particular experiment, or set of experiments, to be carried out upon a sample. In an NMR system for displaying an image having spatial brightness related to the spatial characteristics of one particular nuclear specie, e.g. a proton (.sup.1 H) image, with the sample immersed in a main static magnetic field B.sub.0 of essentially constant magnitude, the NMR excitation/response, or Larmor, frequency F is given in accordance with the well-known relationship: F=.gamma.B.sub.0 /2.pi., where .gamma. is the gyromagnetic constant for that particular nuclear specie. Thus, for a single-specie (proton) imaging NMR system (with .gamma.=42.58 MHz./Tesla, for .sup.1 H) operating at a static field intensity B.sub.0 in the range from about 0.1 Tesla (T) to about 4 T, a single center operating frequency F of between about 4.26 MHz. and about 170.32 MHz., with an effective bandwidth of no more than 1%, would be necessary. It is relatively easy to provide such a system with a relatively low-cost RF preamplifier having a low "spot", or narrow-bandwidth, noise figure. However, in an NMR system for performing both imaging and spectroscopy experiments upon a sample, such as a human patient for medical diagnostic purposes, a single nuclear specie is rarely involved, even if practical considerations do require that the main static magnetic field have essentially a single value. For example, in analyzing living tissue, not only is the proton density (.gamma.=42.58 MHz./T) of interest, but spectroscopic chemical-shift information may be acquired for such nuclear species as: carbon (.sup.13 C), having a gyromagnetic ratio .gamma. of about 10.71 MHz./T; fluorine (.sup.19 F), having a gyromagnetic ratio .gamma. of about 40.05 MHz./T; sodium (.sup.23 Na), having a gyromagnetic ratio .gamma. of about 11.26 MHz./T; and phosphorous (.sup.31 P), having a gyromagnetic ratio .gamma. of about 17.23 MHz./T. It will be seen that the effective receiving system bandwidth and, therefore, the preamplifier bandwidth, is now substantially established by the ratio of the highest to lowest gyromagnetic ratios for the involved nuclear species. Specifically, for the chemical-shift spectrographic imaging NMR system capable of operating with hydrogen, carbon, fluorine, sodium and phosphorous nuclei, a frequency span of almost two octaves (i.e. almost 4.times., or 42.58/10.71=3.976) is required. In such a system (as is described and claimed in co-pending U.S. patent application Ser. No. 743,125, filed June 10, 1985, assigned to the assignee of the present application and incorporated by reference in its entirety herein), a single main static magnetic field having an amplitude between about 0.7 T and about 4 T is utilized for production of proton and chemical-shift spectrographic images, corresponding to a frequency band from about 7.5 MHz. to about 29.8 MHz. for the lowest static field B.sub.0 magnitude (0.7 T) system and a frequency range from about 42.8 MHz. to about 170.3 MHz. for the system using the highest static field B.sub.0 magnitude (4 T). For the exemplary system of the aforementioned co-pending application, in which a main static magnetic field of about 1.5 T is used, the response signal preamplifier must have a low-noise, high-gain bandwidth between at least 16 MHz. and at least 64 MHz., which frequency range will be utilized, by way of example only, in the present application.
Typically, a low-noise RF preamplifier is designed to have its input attached to a transformed signal source impedance having a predetermined optimum value. This preamplifier optimum source impedance is normally established by an input network matching the preamplifier device to a standard RF characteristic impedance Z.sub.0, e.g. Z.sub.0 =50 ohms. In an NMR system, the source impedance seen by the low-noise preamplifier is provided by the response-signal antenna; the antenna impedance will itself depend upon the magnitude and phase of any external loading. It has been found that, due to the differing sizes of the sample-to-be-investigated introduced into the effective volume of the antenna, the antenna impedance can vary over a wide range; this is especially true in systems extracting medical diagnostic data from human patients, where the size, weight and other characteristics of the patient can vary over relatively broad ranges. The result of any deviation from the optimum value of response-signal antenna impedance results in a noise level degradation, which degradation increases with increasing noise factor (F)/noise figure (NF)/noise temperature (T.sub.e) of the reception system. A theoretical and experimental analysis will show that the noise level degradation is reduced, for the same relative ratio of actual source impedance to optimum source impedance, as the noise figure of the receiver is itself reduced. Thus, providing a high-gain, broad bandwidth reception preamplifier having a very low noise level permits the reception antenna source impedance to vary over a conjugately broader range, without requiring additional impedance-matching networks between the antenna terminals and the preamplifier input. The elimination of impedance-matching networks is especially useful in a clinical medical environment, as it saves set-up time while the patient is within the bore of the NMR system magnet and thus reduces the time which both the patient and the imaging staff must spend in obtaining the desired information. In addition, the elimination of an impedance-matching network also removes the network attenuation from appearing between the reception antenna and the preamplifier input; absence of this additional signal loss mechanism itself increases the NMR signal-to-noise ratio.